De Novo Design of Quasi Symmetric Protein Particles

The quasi-equivalence geometric principle elucidates how viral capsids tessellate spherical shells with a combination of 12 pentagons and different numbers of hexagons. However, the rational design of quasi-symmetric capsids through self-assembly remains challenging, since protein subunits undergo subtle conformational changes, assuming multiple non-equivalent local environments that break perfect icosahedral symmetry. Here, we describe a computational design strategy to generate two-component quasi-symmetric protein cages. By creating building blocks that self-assemble into hexagonal motifs with pre-specified positive curvatures, we induce particles to close in a self-limiting manner through the formation of pentameric defects. Electron microscopy analyses reveal that the size of these in vitro assembled cages can be tailored from 40 nm to over 200 nm, confirming that particle closure is facilitated by pentagonal defects as intended. These de novo particles show robust self-assembly and genetic fusion both in vitro and within living cells, paving the way for potential application as de novo virus-like particles and genetically encoded bioprobes. Together, our results lay the groundwork for programmable quasi-symmetric architectures as a new class of protein nanomaterials for a wide range of biomedical applications.